Edge Slide slide vertices too
It seems you're using Grab. With Grab (g) you're simply moving the edge, which is made out of vertices that it shares with the adjacent edges, hence they are also manipulated.
Try using Extrude (e) instead. This will create a new edge (a set of two vertices) in the exact same location that you can manipulate independent of the adjacent geometry.
If you then set the Transformation orientation to 'Local' , you can move it along the axis in the same direction as the original roof: